You are lucky, if you are boarding in Cairo International Airport this is in the middle of Cairo, Nasr City where is the place to go to every awesome place.
If your Layover is 12 hours its plenty of time to take a little bit of Cairo ~,*
Just make sure if your flight is between 8AM to 4PM you need to leave couple hours early the airport road is ALWAYS busy on this time and I mean very busy.
You can looking to go to City Stars, one of the malls that will surprise anyone, huge, very architecture, if you need to buy anything it is in this mall. Things are cheap in Egypt if you converting from Dollars to Egyptian pounds.
You can also go look for the Baron Palace, its building for the great civil engineer of all time Baron Empain its worth visiting.
and in these 12 hours if we say we can take 4 for transportation then you've 8 hours these 8 hours you can see the beautiful pyramids and sphinx at night or daylight.
You can go to old Cairo where everything feels old natural and just have this great spirit to it hard to find words can just express what I would like them to do. So, There is also this gas station on The Autostrad Rd "The Airport road" called Auto-Run and have place for juice called FBI, I knew its ridiculous. Youth of Egypt and even older go to this gas station with well modified cars to race and drag on the street, majority of these people are professional drivers, but yet they are doing dangerous things on the street. Chilling at this place has sort of fun at least for me.
OKAY, This place I mean if you are a fan of food and taste, you need to search and go to Andreas restaurant in Giza. I am Egyptian I know how to cook this food have ate is so many times, but when I've visit this place I said and will forever say This is the best Egyptian food I've ever had. Beyond amazing.
Another way to experience something is extremely awesome, I used to do it all the time when I want to entertain myself, I ride the Metro which is basically a slow speed train that drives inside allover Cairo, which you see different parts of Egypt awesome, but this doesn't go to the airport you can take a taxi. as I said cheap this whole thing won't cost you $50
Hope your layover in Egypt is fun.
BTW, my house is just 15 minutes off the airport I can see it from my balcony. I miss Egypt.